Machines such as punch presses have heretofore been made for operating on sheet stock workpieces and which have stock grippers mounted on a work positioning carriage for moving the sheet stock workpiece along first and second relatively transverse paths in the plane of the workpiece, to position the workpiece at predetermined positions relative to tools which perform operations on the workpiece. It is desirable to enable adjustment of the stock grippers relative to the work positioning carriage to not only accommodate different size sheet stock workpieces, but to also enable gripping of the stock at locations spaced from the areas on the workpiece where machine operations are to be performed.